1. Field of the Invention
This invention relates to polyurethanes which can be produced using polyethers with a particularly narrow molecular weight distribution (polydispersity, PD) or a particular OH functionality or both, to preparations which contain polyurethanes such as these and to the use of the polyurethanes and preparations containing them.
2. Discussion of the Related Art
As multipurpose plastics, polyurethanes are used in many areas of technology, above all in surface coating compositions, adhesives and sealing compounds. Polyurethanes with reactive terminal groups, particularly terminal groups capable of reacting with water, are of particular interest in this regard. This form of reactivity enables the reactive polyurethanes to be brought to the required place in the required processable form, generally liquid or highly viscous, and cured by the addition of water or other compounds reactive with the terminal groups (hardeners, for example in the case of two-component systems). The hardener is generally added before processing, the processing time available to the processor after addition of the hardener being limited. However, polyurethanes containing reactive terminal groups may also be cured solely by reaction with atmospheric moisture, i.e. without the addition of hardeners (one-component systems). One-component systems generally have the advantage over two-component systems that the user is spared the frequently onerous task of mixing the often viscous components before use.
The polyurethanes containing reactive terminal groups commonly used in one-component and two-component systems include, for example, polyurethanes terminated by NCO groups. Given suitable functionality, polyurethanes such as these cure solely under the effect of atmospheric moisture. In certain circumstances, particularly in the presence of water (for example on moist surfaces), the use of NCO-terminated polyurethanes can be accompanied by the evolution of carbon dioxide which can have adverse affects, for example on surface structure. In addition, polyurethanes such as these often do not adhere to smooth inert surfaces, for example to surfaces of glass, ceramics, metals and the like, which in many cases necessitates the use of a primer before application of the polyurethane or makes the polyurethanes impossible to use for sealing, bonding or for sealing a surface of those materials.
In order to remedy this situation, i.e. to enable a firm and durable bond to be established between the polyurethane and, for example, the surfaces mentioned above, it has been proposed to introduce an alkoxysilane group, for example, into the polyurethane as a reactive terminal group.
Thus, CA-B 836 076, for example, describes polymers crosslinkable at room temperature which cure on exposure to atmospheric moisture. Among the polymers described as suitable for this purpose are polymers which contain at least two urethane bonds (polyurethanes), the isocyanate functions originally present having been reacted with an alkoxysilane compound. The alkoxysilane compound contains an isocyanate-reactive hydrogen atom and has at least one hydrolyzable alkoxy group at the Si atom which, although stable under the reaction conditions, can be hydrolyzed by moisture. The polyurethanes described as suitable include, for example, a product of a polyoxyalkylene polyol and an aromatic polyisocyanate.
EP-A 0 170 865 describes a process for the production of coating, sealing and adhesive compositions stable in storage in the absence of moisture. To this end, NCO-terminated polyether urethanes are reacted with amino- or mercaptofunctional alkoxysilanes containing at least two ether oxygen atoms in at least one alkoxy group to form prepolymeric silanes.
DE-A 25 51 275 describes an automobile sealing compound which cures solely via terminal alkoxysilane groups. The end product of this process is extremely hard.
EP-B 0 261 409 describes a process for the production of alkoxysilane-terminated moisture-curing polyurethanes in which diols and/or triols are reacted with diisocyanates to form NCO-terminated polyurethane prepolymers of which the functionality is subsequently adjusted to a value of 1 to 2 and the polyurethane prepolymers thus obtainable are reacted with corresponding organosilicone compounds to form alkoxysilane-terminated polyurethanes.
A general disadvantage of all moisture-curing polyurethanes or preparations (systems) containing them, but especially the silicon-containing systems, is that, after processing, the systems become brittle as a result of the curing process and, accordingly, lose a large part of their elasticity or show poor tear propagation resistances. The loss of elasticity occurs particularly frequently at low temperatures, the known systems often losing their elasticity and flexibility on cooling. Unfortunately, attempts to improve the elasticity and flexibility of the cured systems often result in a deterioration in other properties, for example greater surface tackiness, or alternatively the systems have such a high viscosity before processing that solvents, for example, have to be used to guarantee processability. A reduction in the shelf life of the systems is also observed in many cases. Both the deterioration in key material or storage properties and the use of solvents are economically and ecologically inappropriate.
The term “polyurethane” in the following text stands for a particular polyurethane structure which can be obtained by a controlled single-stage or multistage polyurethane synthesis. The term encompasses any deviations from this structure arising out of the statistical nature of the polyaddition process.
The term “preparation” in the context of the present invention stands for mixtures containing a “polyurethane” or a mixture of two or more “polyurethanes” as defined above and optionally other additives which may either be present from the polyurethane synthesis (solvents, catalysts) or which have been subsequently added to the polyurethane or to the mixture of two or more polyurethanes (for example plasticizers, reactive diluents, fillers and the like).